Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems, alternative energy generation, satellites and electric vehicles.
Modern flywheel energy storage systems convert back and forth between a spinning flywheel's rotational energy and electrical energy. A flywheel energy storage system includes a flywheel, a motor and generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor and generator converts between electrical and mechanical while the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low-pressure enclosure to minimize aerodynamic losses that would occur from atmospheric operation.
Two key advantages of flywheels over electrochemical battery systems are longevity and reliability. Electrochemical batteries, and lead acid batteries in particular, have short lifetimes between two and seven years depending on the operating conditions. These batteries require periodic maintenance, can fail unpredictably and are not environmentally friendly. In contrast, flywheel energy storage systems are expected to have lifetimes of at least 20 years and to require little or no maintenance. Such capability can offset the higher initial costs of the flywheel system over batteries by actually becoming more economical when considered over their life system.
Despite the performance advantages of flywheel systems, to be an economically viable alternative to electrochemical batteries they must be designed to maximize the energy storage capacity while minimizing the cost. Composite material (carbon fiber/epoxy and glass fiber/epoxy) flywheels are actively being pursued as a low cost means of storing kinetic energy. Composites offer the advantages of very high hoop direction strengths. Using recent manufacturing developments, composite flywheels can now also be produced economically. Because composite flywheels develop their strength as the result of bonding together already high strength fibers, they can be made as large as desired with essentially the same strength. However, as a raw material, steel is still much less expensive than composite materials. Unfortunately, problems have existed in efficiently storing large amounts of energy in steel flywheels. Small steel flywheels have successfully been made and rotated to high speeds due to their high strength capability. However, in large steel flywheels, necessary for storing large amounts of energy, on the order of 5-10 kilowatt-hours of energy or more, steel cannot be utilized for a flywheel material as effectively. Although the stresses in large flywheels are the same as stresses in small flywheels spinning with the same peripheral speed, the problem is that the tensile strength of large flywheels is significantly lower. Even with careful selection of the steel and manufacturing process, the strength in a large flywheel can be lower by as much as a factor of two compared to a small flywheel. The phenomenon of lower strength occurring in large steel structures is not unique to flywheels and is common in many applications. However, the loading and other requirements in most applications do not make the lowered strength as problematic.
The strength of steel is directly related to its hardness condition, which results from the internal martensitic concentration in the steel. Hardness in steel is developed and controlled by heat treating of the steel. To gain hardness, the steel is heated to austenizing temperature and then rapidly cooled or quenched. The severity of the quench determines the hardness that the steel will achieve. In thick structures, only the surface can cool at a very high rate while the center material cools slower due to the thermal mass and slow heat transfer rate. Therefore, the larger the steel part and the deeper distance from the surface, the lower the hardness and tensile strength. Beyond a certain depth from the surface, the hardness and strength reaches its lower limit for that type of steel beyond which it does not decrease any further. The ability for a particular type of steel to achieve hardness for a given level of quench severity is known as the steels hardenability. Hardenability is directly related to the percentages of carbon and other alloying elements such as chromium, molybdenum, nickel, etc. Therefore, high alloy steels are usually preferred for large highly stressed parts. The higher hardenability allows the parts to develop a higher strength despite the large part size. It should also be noted that the loading in most structures is primarily bending. The maximum stresses therefore occur at the surfaces of the structure, which inherently corresponds to the highest strength portion of the steel. The depth of hardness in these cases is not a significant issue. After the steel part has been quenched, it is usually too brittle to use, so it is then tempered. Tempering is the process of heating the steel to a temperature lower than austenizing temperature and holding it there for several hours before cooling. The result is some reduction in hardness and tensile strength, but a significant reduction in residual internal stresses and a necessary large increase in toughness are usually gained.
Steel flywheels are unusually stressed structures. For the maximum energy storage density, the flywheel is constructed as a solid cylinder. With this configuration, the radial and hoop direction stresses are both equal and maximum in the center. However, the center is also the place where the steel has the lowest strength, as previously explained by the incapability of obtaining a severe quench at that location. High alloy steels can be used to achieve higher strength at a greater depth from the flywheel surface, but for flywheels greater than several inches in thickness and diameter, the achievable strength at the center and hence energy storage capability is still significantly reduced. For steel flywheels constructed of only 10 inch thickness and diameter, the strength can drop by as much as a factor of two from the surface to the center. The toughness of the steel can also suffer in larger flywheels, because steel that is not fully hardened during quenching cannot be tempered to the highest toughness. High alloy steels and very high alloy tool steels typically utilize oil quenching or even air quenching, which is much less severe than a water quench. An extreme water quench used instead of an oil quench with these steels does not increase the depth of hardening and center strength because the heat transfer rate does not significantly increase after a depth of a few inches below the part surface. The higher quench rate of water with high alloy steels will also generate extreme residual stresses at the surface, possibly cracking the steel round.
On a material cost basis, steels could hold great promise for very low cost energy storage flywheels. In small sizes, steel flywheels currently show exceptional strength and energy storage capability per cost. However, when the flywheel size is increased to store a large amount of energy, the performance unfortunately drops dramatically.